Semiconductor devices such as a CCD and a CMOS image sensor using semiconductor integrated circuit technology are broadly used in a digital camera and a cellular phone having a camera function. As such a semiconductor device, there is suggested an image sensor of a rear surface irradiation type in which a light receiving portion having a light receiving element such as a photodiode is provided on a front surface of a semiconductor substrate and a light receiving surface (light irradiated surface) for the light receiving portion is provided on a rear surface of the semiconductor substrate (see JP-A 2007-013089 (KOKAI)). Since not requiring formation of a wiring or an extra film on the light receiving surface (rear surface of the semiconductor substrate), the rear surface irradiation type image sensor has an advantage that higher sensitivity than in a front surface irradiation type image sensor is possible.
In the rear surface irradiation type image sensor, in order to effectively collect light incident on the rear surface of the semiconductor substrate by the light receiving portion provided on the front surface, the semiconductor substrate is required to be thinned. A thickness of the semiconductor device is required to be made thin to be equal to or thinner than 20 μm in a case of incident of a visible light for example, and further to be equal to or thinner than 10 μm, in order to prevent an electric charge generated on the light receiving surface (rear surface) from diffusing before being collected by the light receiving portion, leading to deterioration of a resolution. A semiconductor device applied to such a rear surface irradiation type image sensor is fabricated as follows for example.
First, a light receiving portion having a light receiving element such as a photodiode and an integrated circuit is formed on a front surface of a semiconductor substrate (semiconductor wafer). Next, a supporting substrate (supporting wafer) of almost the same diameter is joined to the front surface of the semiconductor substrate. The supporting substrate functions as a reinforcing body in forming a light receiving surface by thinning the semiconductor substrate from its rear surface side to a neighborhood of the light receiving portion. Next, an anti-reflection film, a color filter, a light-collecting micro lens and the like are formed on the light receiving surface. Further, an external electrode connected to the integrated circuit of a front surface side is formed on a rear surface of the semiconductor substrate, a joined body of the semiconductor substrate and the supporting substrate is cut by a dicing blade, to be individualized into respective semiconductor devices (semiconductor chips).
In a manufacturing process of such a semiconductor device, a processing is performed by mechanical polishing or chemical mechanical polishing from the rear surface of the semiconductor substrate toward the front surface on which the light receiving portion is provided. With regard to the rear surface irradiation type image sensor, it is desirable that the semiconductor substrate is made as thin as possible. However, as a result that the semiconductor substrate is processed to be thin, a residual stress at a time of formation of the integrated circuit constituted by a metal wiring and an insulating film on the front surface of the semiconductor device is apt to concentrate on a joint surface of the semiconductor substrate and the supporting substrate.
As a joining method of the semiconductor substrate and the supporting substrate, there are known a bonding method using an organic bonding agent, a direct joining method in which a surface of a semiconductor substrate and a surface of a supporting substrate are inorganically joined, and so on. Whichever of the above joining method is applied, there is a problem that peeling is apt to occur from a joint interface in a chip end surface based on a stress concentrating on a joint surface when a joined body is individualized by a dicing blade. Peeling from the joint interface causes occurrence of separation of a semiconductor substrate and a supporting substrate or fracture of the semiconductor substrate, whereby a manufacturing yield of a semiconductor device is deteriorated. Even if a semiconductor device and a supporting substrate are not completely separated, progress of peeling into the inside of a joined body warps a thin semiconductor substrate considerably and deforms a light receiving surface, causing deterioration of an image pick-up characteristic.